[openstreetmap-website] Don't allow characters which aren't valid in XML 1.0. (#1139)

[Tom Hughes]

Apologies for the duplication here. I had literally already done the exact same change and was in the middle of writing a test to go with it when your PR came in last night.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/1139#issuecomment-172802577
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